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Refactor XLogInsert a bit. The rdata entries for backup blocks are now 

constructed before acquiring WALInsertLock, which slightly reduces the time
the lock is held. Although I could not measure any benefit in benchmarks,
the code is more readable this way.
This commit is contained in:
Heikki Linnakangas 2012-01-11 09:46:18 +02:00
parent 26e89e7f23
commit 9c808f89c2
1 changed files with 92 additions and 112 deletions
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204
src/backend/access/transam/xlog.c
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@ -694,6 +694,7 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
uint32 freespace;
int curridx;
XLogRecData *rdt;
XLogRecData *rdt_lastnormal;
Buffer dtbuf[XLR_MAX_BKP_BLOCKS];
bool dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
BkpBlock dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
@ -708,6 +709,7 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
bool updrqst;
bool doPageWrites;
bool isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
uint8 info_orig = info;
/* cross-check on whether we should be here or not */
if (!XLogInsertAllowed())
@ -731,23 +733,18 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
}
/*
* Here we scan the rdata chain, determine which buffers must be backed
* up, and compute the CRC values for the data. Note that the record
* header isn't added into the CRC initially since we don't know the final
* length or info bits quite yet. Thus, the CRC will represent the CRC of
* the whole record in the order "rdata, then backup blocks, then record
* header".
* Here we scan the rdata chain, to determine which buffers must be backed
* up.
*
* We may have to loop back to here if a race condition is detected below.
* We could prevent the race by doing all this work while holding the
* insert lock, but it seems better to avoid doing CRC calculations while
* holding the lock. This means we have to be careful about modifying the
* rdata chain until we know we aren't going to loop back again. The only
* change we allow ourselves to make earlier is to set rdt->data = NULL in
* chain items we have decided we will have to back up the whole buffer
* for. This is OK because we will certainly decide the same thing again
* for those items if we do it over; doing it here saves an extra pass
* over the chain later.
* holding the lock.
*
* We add entries for backup blocks to the chain, so that they don't
* need any special treatment in the critical section where the chunks are
* copied into the WAL buffers. Those entries have to be unlinked from the
* chain if we have to loop back here.
*/
begin:;
for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
@ -764,7 +761,6 @@ begin:;
*/
doPageWrites = fullPageWrites || Insert->forcePageWrites;
INIT_CRC32(rdata_crc);
len = 0;
for (rdt = rdata;;)
{
@ -772,7 +768,6 @@ begin:;
{
/* Simple data, just include it */
len += rdt->len;
COMP_CRC32(rdata_crc, rdt->data, rdt->len);
}
else
{
@ -783,12 +778,12 @@ begin:;
{
/* Buffer already referenced by earlier chain item */
if (dtbuf_bkp[i])
rdt->data = NULL;
else if (rdt->data)
{
len += rdt->len;
COMP_CRC32(rdata_crc, rdt->data, rdt->len);
rdt->data = NULL;
rdt->len = 0;
}
else if (rdt->data)
len += rdt->len;
break;
}
if (dtbuf[i] == InvalidBuffer)
@ -800,12 +795,10 @@ begin:;
{
dtbuf_bkp[i] = true;
rdt->data = NULL;
rdt->len = 0;
}
else if (rdt->data)
{
len += rdt->len;
COMP_CRC32(rdata_crc, rdt->data, rdt->len);
}
break;
}
}
@ -819,39 +812,6 @@ begin:;
rdt = rdt->next;
}
/*
* Now add the backup block headers and data into the CRC
*/
for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
{
if (dtbuf_bkp[i])
{
BkpBlock *bkpb = &(dtbuf_xlg[i]);
char *page;
COMP_CRC32(rdata_crc,
(char *) bkpb,
sizeof(BkpBlock));
page = (char *) BufferGetBlock(dtbuf[i]);
if (bkpb->hole_length == 0)
{
COMP_CRC32(rdata_crc,
page,
BLCKSZ);
}
else
{
/* must skip the hole */
COMP_CRC32(rdata_crc,
page,
bkpb->hole_offset);
COMP_CRC32(rdata_crc,
page + (bkpb->hole_offset + bkpb->hole_length),
BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
}
}
}
/*
* NOTE: We disallow len == 0 because it provides a useful bit of extra
* error checking in ReadRecord. This means that all callers of
@ -862,70 +822,20 @@ begin:;
if (len == 0 && !isLogSwitch)
elog(PANIC, "invalid xlog record length %u", len);
START_CRIT_SECTION();
/* Now wait to get insert lock */
LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
/*
* Check to see if my RedoRecPtr is out of date. If so, may have to go
* back and recompute everything. This can only happen just after a
* checkpoint, so it's better to be slow in this case and fast otherwise.
*
* If we aren't doing full-page writes then RedoRecPtr doesn't actually
* affect the contents of the XLOG record, so we'll update our local copy
* but not force a recomputation.
*/
if (!XLByteEQ(RedoRecPtr, Insert->RedoRecPtr))
{
Assert(XLByteLT(RedoRecPtr, Insert->RedoRecPtr));
RedoRecPtr = Insert->RedoRecPtr;
if (doPageWrites)
{
for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
{
if (dtbuf[i] == InvalidBuffer)
continue;
if (dtbuf_bkp[i] == false &&
XLByteLE(dtbuf_lsn[i], RedoRecPtr))
{
/*
* Oops, this buffer now needs to be backed up, but we
* didn't think so above. Start over.
*/
LWLockRelease(WALInsertLock);
END_CRIT_SECTION();
goto begin;
}
}
}
}
/*
* Also check to see if forcePageWrites was just turned on; if we weren't
* already doing full-page writes then go back and recompute. (If it was
* just turned off, we could recompute the record without full pages, but
* we choose not to bother.)
*/
if (Insert->forcePageWrites && !doPageWrites)
{
/* Oops, must redo it with full-page data */
LWLockRelease(WALInsertLock);
END_CRIT_SECTION();
goto begin;
}
/*
* Make additional rdata chain entries for the backup blocks, so that we
* don't need to special-case them in the write loop. Note that we have
* now irrevocably changed the input rdata chain. At the exit of this
* loop, write_len includes the backup block data.
* don't need to special-case them in the write loop. This modifies the
* original rdata chain, but we keep a pointer to the last regular entry,
* rdt_lastnormal, so that we can undo this if we have to loop back to the
* beginning.
*
* At the exit of this loop, write_len includes the backup block data.
*
* Also set the appropriate info bits to show which buffers were backed
* up. The i'th XLR_SET_BKP_BLOCK bit corresponds to the i'th distinct
* buffer value (ignoring InvalidBuffer) appearing in the rdata chain.
*/
rdt_lastnormal = rdt;
write_len = len;
for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
{
@ -974,6 +884,76 @@ begin:;
}
}
/*
* Calculate CRC of the data, including all the backup blocks
*
* Note that the record header isn't added into the CRC initially since
* we don't know the prev-link yet. Thus, the CRC will represent the CRC
* of the whole record in the order: rdata, then backup blocks, then
* record header.
*/
INIT_CRC32(rdata_crc);
for (rdt = rdata; rdt != NULL; rdt = rdt->next)
COMP_CRC32(rdata_crc, rdt->data, rdt->len);
START_CRIT_SECTION();
/* Now wait to get insert lock */
LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
/*
* Check to see if my RedoRecPtr is out of date. If so, may have to go
* back and recompute everything. This can only happen just after a
* checkpoint, so it's better to be slow in this case and fast otherwise.
*
* If we aren't doing full-page writes then RedoRecPtr doesn't actually
* affect the contents of the XLOG record, so we'll update our local copy
* but not force a recomputation.
*/
if (!XLByteEQ(RedoRecPtr, Insert->RedoRecPtr))
{
Assert(XLByteLT(RedoRecPtr, Insert->RedoRecPtr));
RedoRecPtr = Insert->RedoRecPtr;
if (doPageWrites)
{
for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
{
if (dtbuf[i] == InvalidBuffer)
continue;
if (dtbuf_bkp[i] == false &&
XLByteLE(dtbuf_lsn[i], RedoRecPtr))
{
/*
* Oops, this buffer now needs to be backed up, but we
* didn't think so above. Start over.
*/
LWLockRelease(WALInsertLock);
END_CRIT_SECTION();
rdt_lastnormal->next = NULL;
info = info_orig;
goto begin;
}
}
}
}
/*
* Also check to see if forcePageWrites was just turned on; if we weren't
* already doing full-page writes then go back and recompute. (If it was
* just turned off, we could recompute the record without full pages, but
* we choose not to bother.)
*/
if (Insert->forcePageWrites && !doPageWrites)
{
/* Oops, must redo it with full-page data. */
LWLockRelease(WALInsertLock);
END_CRIT_SECTION();
rdt_lastnormal->next = NULL;
info = info_orig;
goto begin;
}
/*
* If there isn't enough space on the current XLOG page for a record
* header, advance to the next page (leaving the unused space as zeroes).